In recent years, with respect to semiconductor manufacturing facilities, chemical products manufacturing facilities, and the like, pressure type flow rate control apparatuses have been widely utilized to replace mass flow rate controllers. With respect to the pressure type flow rate control apparatuses, a so-called “metal diaphragm control valve” has been employed with increasing frequency due to its many advantages, such as high corrosion resistance, a low dust producing nature, excellent gas displacement property, fast open/close velocity, and the like. Also, piezoelectric element driven actuators having the characteristics of great driving force, excellent responsivity and control characteristics have been widely used as the actuator for pressure type flow rate control apparatuses.
Conventionally known metal diaphragm control valves employing a piezoelectric element driven actuator include ones provided with the structure disclosed in Japanese Unexamined Patent Application No. 7-310842 (Patent Document 1) and Japanese Unexamined Patent Application No. 2004-197754 (Patent Document 2), and the like.
More specifically, a control valve (not illustrated) as disclosed in Patent Document 1 is a normally open type control valve wherein a metal diaphragm is thrust to the valve seat side through mediation of an under-side rest, a ball and a diaphragm presser. The diaphragm presser is a piezoelectric element that is elongated with the application of voltage, thus making the metal diaphragm touch a valve seat so as to be in a state of closing, and the length of the elongated piezoelectric element then returns to its original length when voltage applied to the metal diaphragm is switched off thereby causing the thrust applied to the metal diaphragm to clear off. In this way, the metal diaphragm gets back to its original state with the aid of its elastic force; thus, the normally open type control valve returns to a state of being open because the metal diaphragm departs from the valve seat.
With respect to the control valve, the generating force of the piezoelectric element is transmitted to the metal diaphragm through the mediation of the under-side rest, the ball and the diaphragm presser. In this way, the generating force of the piezoelectric element is made to be applied directly to the seat part (comprising a valve seat and a portion contacting with the seat of a metal diaphragm) due to elongation of the piezoelectric element once the diaphragm touches the valve seat. As a result, the force applied to the seat part of the control valve depends only on the generating force of the piezoelectric element. This configuration makes adjustment difficult, and also causes disadvantages in that the metal diaphragm, the valve seat, and the like, might be damaged because a large generating force generated by the piezoelectric element is applied to the seat part.
On the other hand, a control valve (not illustrated) such as is disclosed in Patent Document 2 is a normally open type control valve with which the displacement length of a piezoelectric element is lengthened by means of a displacement lengthening mechanism having a lever structure. This piezoelectric element (a piezo actuator) employed by the control valve becomes elongated with the application of voltage, which places the control valve in the state of being closed (i.e., closed state) as the metal diaphragm of the control valve is thrust and contacts with a valve seat side through mediation of a valve shaft and a diaphragm presser. On the other hand, once the piezoelectric element is in a state of elongation (i.e., the closed state), it returns to its original length, and the displacement lengthening mechanism thus returns to its original state by means of a return elastic body provided thereon. In this way, the thrust force applied to the metal diaphragm is cleared off (i.e., ceases) and the metal diaphragm returns to its original state with the aid of an elastic force provided by the return elastic body so that the control valve moves to the state of being open (i.e., the open state) by means of the diaphragm departing from the valve seat.
The control valve, therefore, is equipped with a constant pressure elastic body (a shock absorbing conical spring) that absorbs elongation of the piezoelectric element when the displacement lengthening mechanism transmits the generating force provided by the piezoelectric element to the metal diaphragm. Then, once the metal diaphragm makes contact with the valve seat, the elastic body, which exerts a constant pressure, absorbs elongation of the piezoelectric element. In this way, a repulsion force is applied to the seat part in a manner corresponding to the force of the displacement length of the constant pressure elastic body. As a result, the force applied to the seat part of the control valve becomes the repulsion force of the constant pressure elastic body, which allows the metal diaphragm to make contact with the valve seat while absorbing shock. This prevents the metal diaphragm and the valve seat from being damaged without the piezoelectric element having to exert a large generating force to the metal diaphragm and the valve seat.
Now, with respect to a control valve that is not provided with a shock absorbing conical spring, for example, the following observation is made. In the case when a displacement length per 1V of applied voltage of the piezoelectric element is 0.333 μm and the generating force per 1V of applied voltage of the piezoelectric element is approximately 5N, then the ratio of generating force to displacement length of the piezoelectric element becomes 5/0.333≈15 N/μm. On the other hand, in the case when the control valve is provided with a shock absorbing conical spring, the ratio of generating force to displacement length of the conical spring becomes approximately 0.267 N/μm because of the spring rate of the conical spring. Accordingly, the force applied to the seat part of the control valve provided with a shock absorbing conical spring results in approximately 1/56th the force applied to the seat part of the control valve not provided with a shock absorbing conical spring. As a result of this difference, damage to the metal diaphragm and to the valve seat can be prevented by using a control valve provided with a conical spring.
FIG. 6 is a graphic plot of the relationship between the load to a seat part of a control valve and the voltage applied to the piezoelectric element. As is apparent from the graph shown in FIG. 6, it is learned that the load exerted to the seat part is very light for a control valve provided with a conical spring in comparison with a control valve that is not provided with a conical spring.
However, even with a control valve provided with a conical spring, there remain some disadvantages which need to be solved. Specifically, in the case wherein the control valve is used under high temperature conditions, such as 100° C. or higher, it is observed that due to thermal expansion of the supporting cylinder-shaped actuator box there is formed a clearance space between the upper end part of the piezoelectric element and an adjustment cap nut screw-fixed to the upper end part of the supporting cylindrical body of the control valve. This clearance space makes it difficult for the generating force to be transmitted surely and smoothly to the metal diaphragm at the time when the piezoelectric element is elongated, thus making accurate flow rate control impossible or difficult to achieve. It should be especially noted that a control valve's flow rate characteristics are immensely affected by even small thermal expansion of members (e.g., the supporting cylindrical body, and the like) that are components of the control valve because the displacement length of the piezoelectric element is very small.
To solve such a problem as stated above (that is, wherein a clearance has occurred due to the thermal expansion of the supporting cylindrical body), it is preferred that the control valve is structured so that a compressive force of 200N or so can be applied to the piezoelectric element from the outside (i.e., externally). However, such a control valve has not yet been developed to this date.
In accordance with a control valve disclosed in Japanese Unexamined Patent Application Publication No. 2004-197754 (Patent Document 2) and a control valve disclosed in Japanese Unexamined Patent Application Publication No. 2-203087 (Patent Document 3), a returning elastic body and a load spring have been disclosed. However, it is noted that the returning elastic body and the load spring employed by these control valves are both for returning members (i.e., a displacement lengthening mechanism and a valve shaft) to the original position. Also, the returning elastic body and the load spring are both installed inside the control valve. Therefore, in the event that a returning elastic body and a load spring having a large elastic force are employed, it becomes inevitable that the control valve needs to be upsized. Also, another disadvantage with a control valve provided with a returning elastic body and a load spring inside the valve is that it is required that the control valve needs to be dismantled in order to replace the returning elastic body or the load spring, which are stored inside the control valve with another replacement returning elastic body or load spring, or to adjust the elasticity of the returning elastic body or load spring. Assembling and disassembling of the control valve are very time-consuming.
[Patent Document 1] Japanese Unexamined Patent Application No. 7-310842.
[Patent Document 2] Japanese Unexamined Patent Application No. 2004-197754.
[Patent Document 3] Japanese Unexamined Patent Application No. 2-203087.